1. Technical Field
The present invention generally relates to a method for improving the viewing quality of CRT display image without the need to increase the resolution of the CRT display, or the display memory storage space. More specifically, characters, appearing in a CRT display image, are apparently positioned at sub-pixel locations to improve the viewing quality. This apparent positioning is accomplished by changing intensity values of certain of the pixels forming characters to be shifted to second intensity values.
2. Description of the Prior Art
Images containing several characters from high resolution printers are often displayed in CRT displays of lower resolution. The characters from the printers typically come from high resolution fonts designed for the high resolution of the printer. Information as to where to position characters on the CRT display field are from printer file formats which contain address locations on a printer display field. Interpreting high resolution file formats results in command signals which are designed for the resolution of the printer and not for the low resolution of the CRT display. That is, these commands contain sub-pixel (see below) address locations and pixel intensity values. Thus, these command signals translate into commands to position characters on the CRT display at sub-pixel locations, i.e., at locations between, and not at, either discrete horizontal or vertical locations of a low resolution CRT display field. Thus, the CRT would follow these commands by rounding off to the nearest pixel location (i.e., at a discrete horizontal and vertical location of a CRT display field), often resulting in erroneous and annoying character spacings on the CRT display.
Throughout this application, unless otherwise indicated, the term "intensity value" will refer to intensity values assigned to CRT pixels. Likewise, the terms "pixel", "pixel locations" or "sub-pixel locations" shall refer, respectively, to CRT pixels or locations on the CRT display field.
Various methods have been used to place characters, from fonts designed for a high resolution bi-level display, onto a lower resolution multi-level display through the use of grey scale techniques. With these techniques, many bi-level intensity values in a number of relatively smaller (in area) bi-level pixels are replaced by a single multi-level intensity value in a relatively larger multi-level pixel. That is to say, the many bi-level intensity values have been replaced by a low resolution (or grey scale) representation. These grey scale techniques are also referred to as anti-aliasing and are discussed by F. C. Crow in a thesis entitled "The Aliasing Problem in Computer Synthesized Shaded Images", University of Utah, March 1976. Various grey scale techniques have also been used to obtain low resolution representations of characters in a font. U.S. Pat. No. 4,158,200 to Seitz et al discusses a method to facilitate the display of grey scale representations of characters in a particular font. In Seitz, a character generator stores signals representing the characters to be displayed. The signals are in binary form and represent multi-level intensity values or levels of grey scale. U.S. Pat. No. 4,385,293 to Wisnieff discloses the use of grey scale levels at discrete points of an AC plasma panel, wherein the grey scale levels are stored in binary form in shift registers. Finally, John E. Warnock discusses storing grey-scale or low resolution representations of characters from a particular font in memory in an article entitled: "The Display of Characters Using Grey Level Sample Arrays". (Computer Graphics SIGGRAPH'80 Conference Proceedings July 1980). In this article, Warnock also discusses storing several different versions of each character, each version representing a different apparent sub-pixel positioning of the character. However, this method requires a large CRT display memory storage space. For example, in a typical case, where the resolution of the printer display is about 8000 pixels per character and the CRT display about 80 pixels per character; 100 different character definitions for each character would have to be stored in memory.
There is need, therefore, for a simple method to improve the viewing quality in a CRT display image by apparently positioning characters appearing therein at sub-pixel locations. This positioning must occur without the expense of increasing either pixel resolution in the CRT display or CRT memory storage space. This need is particularly apparent when characters, of an image from a relatively higher resolution printer display, are formed in a CRT display of relatively lower resolution.